I’ve had plantar fasciitis and know how excruciatingly painful and debilitating it is. I went for the (expensive) high-arch custom orthotics route, but it didn’t help much.
What really did help me was simply to loosen the laces on my boots along the upper above the arch (but tight at the ankle) so my feet could flex back and forth to their natural arched shape when walking. I found that the tight lacing was permanently flattening my feet to the point where it was putting a strain on the plantar fascia ligament underneath, causing the problem.
Try walking around the house/office barefoot when you can. If the pain subsides or doesn’t get any worse, then it’s a good bet that tight lacing will be the culprit.